The inventive subject matter relates to motor drive apparatus and, more particularly, to motor drive apparatus for subsea applications, such as offshore oil and gas exploration.
Motor drives are used in a number of different industrial applications. Variable speed AC motor drives, such as variable frequency drives (VFDs), are used, for example, for compressors, pumps, and the like. A common use of VFDs is to control compressor and pump motors in oil and gas exploration and production operations.
A VFD may control the speed and/or torque of an AC motor by varying the frequency and amplitude of the voltage applied to the motor. A VFD may be used to operate a motor at an optimal speed for a given load and/or to provide a gradual startup and/or slowdown that limits current and protects mechanical components, such as gear trains, pipelines, and the like, from mechanical shock.
A typical VFD may include an inverter that is coupled to a DC power source, which may be a DC power distribution system or DC power derived from an AC power distribution system using a rectifier. The inverter may include semiconductor switches (e.g., transistors, silicon controlled rectifiers and the like) that are controlled by a digital control circuitry including a microcontroller or other data processing device. In particular, the microcontroller may receive feedback signals (e.g., speed, voltage, current and the like) and may responsively generate drive signals that are used to control power transistors or other semiconductor switching devices that are used to drive the motor.
In conventional offshore oil and gas applications, VFDs typically are used to drive motors located on above-surface exploration and production platforms. In recent years, however, oil and gas processing equipment has increasingly moved from such platforms to the seabed. Such subsea processing equipment may be deployed thousands of feet below the surface and may be subjected to intense pressures and/or temperatures that can degrade reliability. They may also be distributed over a wide area. It may be time-consuming and expensive to maintain and replace equipment in this environment due to the depths and distances involved.